Technical Field
The present disclosure relates to a sensing circuit and to a method of detecting an electrical signal generated by a microphone.
Description of the Related Art
As is known, capacitive microphones, in particular MEMS (microelectromechanical systems) microphones and the ECMs (electret capacitor microphones), generally comprise a deformable conductive membrane capacitively coupled to a fixed electrode or plate, also referred to as “back-plate”. Present between the membrane and the fixed electrode is a space, occupied by air.
Commonly available capacitive microphones are biased with a fixed charge and produce an electrical signal in response to acoustic signals defined by pressure waves. In practice, the membrane vibrates in response to acoustic signals modifying the capacitance of the capacitor and, given that the charge stored is fixed, the vibration causes a corresponding voltage variation between the electrodes of the capacitor itself. The electrical signal is defined by the voltage variations on the capacitor. An equivalent circuit normally used for representing a capacitive microphone comprises a capacitor with variable capacitance in series to a voltage generator.
In order to prevent perturbation of the charge stored on the capacitor, the electrical signal produced by the microphone is typically read using a sensing circuit (or “buffer”) with high input impedance, which drives an external load with a low-impedance output, on the basis of the signal detected.
Sensing circuits most commonly used adopt a MOS transistor in source-follower configuration or, alternatively, a class-AB amplifier.
Sensing circuits based upon a source follower are extremely simple and present a low noise, but prove to be effective only to drive high resistive loads (for example, higher than 100 kΩ). Sensing circuits of this type, in fact, may absorb high currents from (supply high currents to) the load, but, instead, are limited in supplying (absorbing) a current not higher than the bias current of the follower transistor. If the load resistance is modest, prior art circuits typically use a rather high bias current to obtain a sufficient output swing, and this involves an unacceptable increase in static consumption, in the absence of a signal.
Sensing circuits based upon class-AB amplifiers are commonly used for driving low loads, for example lower than 10 kΩ. As compared to source-follower sensing circuits, class-AB amplifiers enable supply of high currents when so desired, maintaining limited consumption levels when the input signal is low or absent. However, class-AB amplifiers are much more complex than source-follower circuits and contain several noise sources. Furthermore, to prevent possible oscillations or instability, a rather elaborate frequency compensation is frequently employed. In practice, also the static consumption is not in general as low as would be desirable.